Strong Zero-Phonon Transition from Point Defect-Stacking Fault Complexes in Silicon Carbide Nanowires.

Crystallographic defects such as vacancies and stacking faults engineer electronic band structure at the atomic level and create zero- and two-dimensional quantum structures in crystals. The combination of these point and planar defects can generate a new type of defect complex system. Here, we investigate silicon carbide nanowires that host point defects near stacking faults. These point-planar defect complexes in the nanowire exhibit outstanding optical properties of high-brightness single photons (>360 kcounts/s), a fast recombination time (<1 ns), and a high Debye-Waller factor (>50%). These distinct optical properties of coupled point-planar defects lead to an unusually strong zero-phonon transition, essential for achieving highly efficient quantum interactions between multiple qubits. Our findings can be extended to other defects in various materials and therefore offer a new perspective for engineering defect qubits.

First-Principles Study of Charge Diffusion between Proximate Solid-State Qubits and Its Implications on Sensor Applications.

Solid-state qubits from paramagnetic point defects in solids are promising platforms to realize quantum networks and novel nanoscale sensors. Recent advances in materials engineering make it possible to create proximate qubits in solids that might interact with each other, leading to electron spin or charge fluctuation. Here we develop a method to calculate the tunneling-mediated charge diffusion between point defects from first principles and apply it to nitrogen-vacancy (NV) qubits in diamond. The calculated tunneling rates are in quantitative agreement with previous experimental data. Our results suggest that proximate neutral and negatively charged NV defect pairs can form a NV-NV molecule. A tunneling-mediated model for the source of decoherence of the near-surface NV qubits is developed based on our findings on the interacting qubits in diamond.

Room temperature solid-state quantum emitters in the telecom range.

On-demand, single-photon emitters (SPEs) play a key role across a broad range of quantum technologies. In quantum networks and quantum key distribution protocols, where photons are used as flying qubits, telecom wavelength operation is preferred because of the reduced fiber loss. However, despite the tremendous efforts to develop various triggered SPE platforms, a robust source of triggered SPEs operating at room temperature and the telecom wavelength is still missing. We report a triggered, optically stable, room temperature solid-state SPE operating at telecom wavelengths. The emitters exhibit high photon purity (~5% multiphoton events) and a record-high brightness of ~1.5 MHz. The emission is attributed to localized defects in a gallium nitride (GaN) crystal. The high-performance SPEs embedded in a technologically mature semiconductor are promising for on-chip quantum simulators and practical quantum communication technologies.

Bright Room-Temperature Single-Photon Emission from Defects in Gallium Nitride.

Room-temperature quantum emitters in gallium nitride (GaN) are reported. The emitters originate from cubic inclusions in hexagonal lattice and exhibit narrowband luminescence in the red spectral range. The sources are found in different GaN substrates, and therefore are promising for scalable quantum technologies.

The spin-spin zero-field splitting tensor in the projector-augmented-wave method.

The projector-augmented-wave (PAW) method of electronic calculations has the unique feature that, while it is basically a pseudopotential-like treatment of density functional theory, it keeps a connection to reconstructible all-electron wavefunctions, and thus it can be considered less an approximation, yet having generally the same computational cost when it is compared to pseudopotential methods. The zero-field splitting of spin-spin interaction energy, although it is a core instrument in the treatment of a wide range of problems involving the behavior of electron spins in various materials, has not yet been implemented in PAW. We provide here all the background considerations for implementing the calculation of the zero-field splitting tensor within the framework of PAW.

Quantum-confined single photon emission at room temperature from SiC tetrapods.

Controlled engineering of isolated solid state quantum systems is one of the most prominent goals in modern nanotechnology. In this letter we demonstrate a previously unknown quantum system namely silicon carbide tetrapods. The tetrapods have a cubic polytype core (3C) and hexagonal polytype legs (4H)--a geometry that creates spontaneous polarization within a single tetrapod. Modeling of the tetrapod structures predicts that a bound exciton should exist at the 3C-4H interface. The simulations are confirmed by the observation of fully polarized and narrowband single photon emission from the tetrapods at room temperature. The single photon emission provides important insights into understanding the quantum confinement effects in non-spherical nanostructures. Our results pave the way to a new class of crystal phase nanomaterials that exhibit single photon emission at room temperature and therefore are suitable for sensing, quantum information and nanophotonics.

Parametrization of the SCC-DFTB Method for Halogens.

Parametrization of the approximative DFT method SCC-DFTB for halogen elements is presented. The new parameter set is intended to describe halogenated organic as well as inorganic molecules, and it is compatible with the established parametrization of SCC-DFTB for carbon, hydrogen, oxygen, and nitrogen. The performance of the parameter set is tested on a representative set of molecules and discussed.

Readout and control of a single nuclear spin with a metastable electron spin ancilla.

Electron and nuclear spins associated with point defects in insulators are promising systems for solid-state quantum technology. The electron spin is usually used for readout and addressing, and nuclear spins are used as exquisite quantum bits and memory systems. With these systems, single-shot readout of single nuclear spins as well as entanglement, aided by the electron spin, have been shown. Although the electron spin in this example is essential for readout, it usually limits the nuclear spin coherence, leading to a quest for defects with spin-free ground states. Here, we isolate a hitherto unidentified defect in diamond and use it at room temperature to demonstrate optical spin polarization and readout with exceptionally high contrast (up to 45%), coherent manipulation of an individual excited triplet state spin, and coherent nuclear spin manipulation using the triplet electron spin as a metastable ancilla. We demonstrate nuclear magnetic resonance and Rabi oscillations of the uncoupled nuclear spin in the spin-free electronic ground state. Our study demonstrates that nuclei coupled to single metastable electron spins are useful quantum systems with long memory times, in spite of electronic relaxation processes.

Automated Repulsive Parametrization for the DFTB Method.

The density-functional-based tight-binding method is an efficient scheme for quantum mechanical atomistic simulations. While the most relevant part of the chemical energies is calculated within a DFT-like scheme, a fitted correction function-the repulsive energy-is used to achieve results as close to ab initio counterparts as possible. We have developed an automatic parametrization scheme to ease the process of the repulsive energy fitting, offering a more systematic and much faster alternative to the traditional fitting process. The quality of the resulting repulsives can be tuned by selecting and weighting the fit systems and the important physical properties (energy, force, Hessian) of them. Besides driving DFT calculators in the fitting process automatically, the flexibility of our scheme also allows the usage of external data (e.g., molecular dynamics trajectories or experimental data) as a reference. Results with several elements show that our procedure is able to produce parameter sets comparable to handmade ones, yet requiring far less human effort and time.